Stainless steel

ABSTRACT

AN AUSTENITIC STAINLESS STEEL HAVING EXCELLENT MACHINABILITY COUPLED WITH GOOD CORROSION-RESISTANCE HAS A COMPOSITION FALLING WITHIN THE FOLLOWING LIMITS, IN WEIGHTS PERCENTAGES:   PERCENT C UP TO 0.1 CR 10-20 NI 10-15 MO 1-5   AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF S 0.1-0.9 AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF TI 0.2-1.5 PB 0.1-0.9 SE 0.1-0.9 TE 0.1-0.9 ZR 0.2-1.5 CU 0-4 SI UP TO 1 MN UP TO 2 FE (1)   1 BALANCE SAVE FOR NORMALLY PRESENT IMPURITIES.

Oct. 9, 1973 J QTROSEUUS ETAL 3,764,302

STAINLESS STEEL.

Filed Nov. 1, 1971 Fig.2

United States Patent ()1 ice 3,764,302 Patented Oct. 9, 1973 3,764,302 STAINLESS STEEL Lars Johan Berti! Troselius, Fagersta, and Claes Ulf Casimir Sparre and Roland Richard Kiessling, Sandviken, Sweden, assignors to Sandvik Aktiebolag, Sandviken, Sweden Filed Nov. 1, 1971, Ser. No. 194,154

Claims priority, application Sweden, Nov. 12, 1970,

US. Cl. 75-125 6 Claims ABSTRACT OF THE DISCLOSURE An austenitic stainless steel having excellent machinability coupled with good corrosion-resistance has a composition falling within the following limits, in weights percentages:

Balance save for normally present impurities.

The present invention relates to a stainless steel alloy and more particularly to an acid-resistant free-machining steel having an essentially austenitic structure.

A demand for good machinability, when cutting in automatic machines, is to an increased extent made upon different materials as for instance steel. This property can usually become improved, and desired machinability be obtained, by small additions of diiferent elements to the steel, as for instance sulphur, lead, tellurium, etc. In cases where such so-called free-machining steels consist of stainless steels, there is a desire that the corrosion resistance shall not be decreased to any appreciable extent by the additions.

It heretofore had been known that the machinability of (for instance) an austenitic stainless steel can be improved by an addition of sulphur. The amounts of sulphur which are needed for satisfactory machinability usually decrease the corrosion resistance very much, however. By addition of other elements, such principally as copper, it has been possible to maintain the corrosion resistance to a certain extent without renouncing the machinability enhancement.

For so-called acid-resistant steels, i.e. austenitic stainless steels having improved corrosion resistance because of an addition of molybdenum in contents up to about 5%, there heretofore have been no qualities or grades which have fulfilled the requirements for sufficient machinability in automatic machines as well as the requirements for maintenance of corrosion resistance.

It has now been found possible, according to the invention, to obtain such a steel alloy which is characterized by a particularly good corrosion resistance in combination with excellent machinability in chipforming machinmg.

The steel according to the invention contains, in weight percentages, from traces up to 0.1% C, 10-20% Cr, 10- 15% Ni, 1-5% Mo, at least 0.1% S and/or Pb and/or Se and/ or Te, the sum of the contents of these elements being 0.1-0.9%, at least 0.2% Ti and/or Zr the sum of the contents of these elements being 0.2-1.5%, at the most 4% Cu, at the most 1% Si, at the most 2% Mn, the rest being iron besides normally present impurities.

Usually the Mo-content is at the lowest 1.5% and preferably at the most 4.0%. Certain high grade steels according to the invention contain preferably at the most 0.05% C.

Compared to conventional acid-resistant steels (such, for instance, as the standardized steels SIS 2343 or AISI 316), the steel according to the invention has well adjusted additions of the elements S respectively Pb, Se or Te and of Ti respectively Zr and preferably also of Cu.

We found that an addition of sulphur gave improved machinability but decreased corrosion resistance, synonymously with the case for several other kinds of stainless steels. An addition of Cu to the steel containing S gave an improvement of the corrosion resistance, but after an addition of Ti also it was possible to ascertain an essential quality improvement according to the invention. It has also been found that an addition of Ti, only, to the steel containing S causes a considerable improvement of the corrosion resistance to essential parts. Analogous effects were observed in adding Cu and Ti or merely Ti to steels containing Pb, Se or Te; also, in adding Cu and Zr or merely Zr to steels containing S, Pb, Se or Te.

In possible explanation of the greatly improved corrosion resistance of the steel according to the invention, the following may be briefly mentioned. In adding merely S, this element forms mainly MnS, which latter has a low corrosion resistance. M is, thus, dissolved in acids and the H 8 which thereby is formed catalyses the cathodeand anode processes. On the other hand, addition of Ti (respectively Zr) gives titanium sulphides (respectively zirconium sulphides) which are much more difiiculty soluble than manganese sulphide in acids, thus causing improved corrosion resistance. A particularly well adjusted addition is 0.5-1.2% as a total content of Ti and/or Zr, preferably 0.8-1.1 (in weight percentages).

The favorable effect particularly concerning the resistance to pitting which has been found by a certain addition of Cu in combination with Ti respectively Zr, is probably connected with deposition of CuS (in case of a sulphur alloyed steel) on the metal surface or immediately adjacent to the surface and to an enrichment of copper on the active steel surface, by which phenomenon the speed of corrosion decreases. It has then been found that the Cu content shall suitably be 03-35%. The content of Cu should preferably be at the lowest 1.0% and at the most 3.0%.

As was mentioned earlier, an addition of S respectively Pb, Se or Te in contents, which are contained by the invention, has given essentially improved machinability. A particularly well adjusted content, in respect to these properties as well as other important qualities of the steel,

3 has been shown to be 02-04% as a total Se, Pb and/or Te.

The additions of Cu and Ti (respectively Zr) which have been found necessary to maintain the original corrosion resistance of the acid-resistant steel have, as mentioned earlier, not caused any decrease of the machinability improvement obtained by the addition of sulphur, respectively lead, selenium or tellurium. In certain cases it has been found necessary carefully to control and also to adjust the Mn-content of the steel. A suitable range of levels of Mn has then been ascertained to be at least 1.0% and preferably at least 1.5% Mn.

In the practical manufacturing of details in automatic machines the wear of the tool is a certain standard gauge of the suitability of diiferent steels for such cutting. Still more important, however, is the strength or toughness of the chips of the material. Too high a strength of the chips, which means bad chipbreaking, may prevent manufacturing of details in automatic machines because of tool breakage and troublesome chip handling. It has been found that the steel according to the invention has remarkably good working properties because of a very low toughness of the chips.

The following examples illustrate the high corrosion resistance, in combination with the excellent machinability, which have been found in steel alloys according to the invention. The examples show the effects of the different mentioned additional elements upon a steel of mainly standard type.

The test included the following variants having analyses according to Table I:

content of S,

The machinability tests specimens were solution-heattreated at 1050 C. and quenched before the test which latter included turning with feed 0.05 mm./rev. and cutting depth 0.75 mm. The total tool material was high speed steel having a hardness of HRC 65. No cutting medium was used. The result in the form of a wear diagram (vTcurves) is presented in FIG. 1. From the diagram it is obvious that the addition of S caused a considerably increased tool life at the cutting operation. It is also seen that the additions of Cu and Ti, which improved the corrosion resistance, did not have any essential efiect on the improvement obtained by the addition of S.

The toughness of the chips were determined by a method illustrated in FIG. 2. The chip was bent first downwards and then upwards, in which the mark I meant breakage in the sector 90", the mark 2 breakage in the sector 90-180", the mark 3 breakage in the sector 180- 90 and so on according to FIG. 2. The result is evident from Table III.

TABLE III Note Steel N 0.

Mark

It was found, thus, that the merely sulphur-alloyed steel TABLE I Si Mn P 0 Cr Ni Mo Cu Ti N No 0.0 1.7 0. 008 0.011 17.3 13.5 2.7 01" Corr. toAISI 310. 0.5 1.0 0. 000 0.20 17.0 13.3 2.0 2.4 0 8 0.1" AISI s10+s+ou+T1. 0.5 1.5 0.010 0.10 17.0 13.7 2.0 2.3 01" AISI 316+S+Cu. 0.0 1.8 0.003 0.21 17.4 13.5 2.7 01" A1S1a16+g 0.0 1.0 0. 008 0.10 17.0 13.0 2.8 1 0 0.1" AISI 310 -24-10.

The preparation of the material included first melting and casting, after which the ingots were heated up to 1200 C. and hot worked to round bar having a diameter of 22 respectively 15 millimeters and flat bar steel having a cross-sectional dimension of 24 X 6 millimeters. From these billets there were made different kinds of test specimeans good corrosion resistance.) From the table it is The corrosion tests included measuring of polarization, passivity and weight loss.

The measurements of polarization included taking up polarization curves in 1 Molar H 80, at 25 C. by means of potentiodynamic measurements. In Table II the parameter Icrit. which is important for stainless steels, is mentioned for the different steel variants. (Low Incrit. means good corrosion resistance). From the table it is obvious that Incrit. has been increased more than 15 times by addition of 0.2% S (steel No. 4 compared to steel No. 1). An addition of copper alone (see steel No. 3) was not suflicient to neutralize the increase of the dissolving speed caused by the sulphur. By addition of about 1% Ti according to the invention (see steels Nos. 2 and 5) it was possible to restore the passivity properties.

The other measurements gave generally similar results. Thus, the weight loss was measured in sulphuric acid solutions of different concentrations at 50 C. and at boiling temperature during 24 hours. The results after measurements in 1 M H 50 50 C. are illustrated in Table H.

1 Thickness corroded in one year.

as Well as the steel variants according to the invention gave brittle, easily separable, chips while the standard steel gave tough chips and therefore was unsuitable for cutting in automatic machines.

Another test included a steel variant having the following composition, in percentages by weight: 0.05% C, 0.6% Si, 1.7% Mn, up to 0.030% P, up to 0.030%5, 17.4% Cr, 13.4% Ni, 2.7% M0, 2.5% Cu, 0.8% Ti, 0.25% Se and the rest principally Fe.

The corrosion tests proved that the mentioned variant had a corrosion resistance which was similar to, or somewhat increased compared to, the resistance of the standard steel (AISI 316). The addition of Se caused at the same time a considerable improvement of tool lifeas Well as of chip separation compared to the standard steel.

It was found that in case of Se as a machinability improving element, a content of (MO-0.50% gave optimum properties of the steel.

When tellurium or lead was chosen as machinability improving elements, it was also found that contents of 0.10-0.50% gave optimum results in the steel.

We claim:

1. Acid-resistant, free-machining, stainless steel having high corrosion resistance combined with excellent machinability in cutting in automatic machines and having essentially austenitic structure, consisting essentially of the following composition in percentages by weight: from a trace up to 0.1% C; 10-20% Cr; 10-15% Ni; 15% M0; at least 0.1% of at least one member of the group consisting of S, Pb, Se and Te, the sum of the contents of these elements being 0.1-0.9%; at least 0.2% of at least one member of the group consisting of Ti and Zr the sum of the contents of these elements being 0.8-1.1%; 0.3-

3.5% Cu, at most 1% Si, at most 2% Mn, the balance being iron besides normally present impurities.

2. Steel according to claim 1 wherein the carbon content is at most 0.05%. 2,215,734 3. Steel according to claim 1 wherein the molybdenum 5 2,519,406 content is from 1.5% to 4.0%. 3177577 2,310,341

4. Steel according to claim 1, wherein the copper con- 3 331 715 tent is 1.0-3.0%. 3:522:03!

5. Steel according to claim 1, wherein the total content 10 of S, Se, Pb and Te is 02-04% 6. Steel according to claim 1, wherein the manganese content is at least 1.0% and preferably at least 1.5%.

References Cited UNITED STATES PATENTS Moskowitz 75-128 P Harder 75128 P Scott 75128 T Fujimura 75--128 T Arness 75-128 W Buling 75-425 Greene 75128 W HYLAND BIZOT, Primary Examiner US. Cl. X.R.

75128 P, 128 T, 128 Z 

